The accurate and sensitive analysis of complicated gas mixtures has many applications in industry, process control, forensics, pharmacology, medicine, and environmental science.
Mixture analysis is also an essential tool in biological, chemical, and physics research. Examples of unsolved, or imperfectly solved, problems are easy to find: over 1000 compounds are known to exist in human breath, including compounds which are markers of health risks, cancer, and environmental hazards; a wide variety of organic amines and carboxylic acids, present in nanomolar concentrations in seawater, are important intermediates in decomposition processes and provide a quantitative measure of marine ecosystem metabolic cycles; the pheromones of butterflies and moths include at least hundreds of different compounds, many of them isomers differing only by the position of a double bond; and human blood and urine contain at least dozens and likely hundreds of biologically important hormones.
Complex gas mixtures are generally analyzed using ion mobility spectrometry, gas chromatography (GC), and gas chromatography mass spectrometry (GC/MS). In gas chromatography, a gas sample to be analyzed is mixed with an inert carrier gas, such as helium or nitrogen, and flowed through a capillary containing a liquid or polymer coated substrate. Different contaminants elute at different times, effectively sorting the contaminants by time as they exit the end of the capillary column. GC alone can resolve a mixture component with a fraction as low as 10−9 (parts per billion range) in specialized cases, but generally operates in the 10−6 range or lower. For complex mixtures, GC is often combined with mass spectrometry (GC/MS) to further resolve mixture components. In this case, molecules are ionized as they elute from the GC column and the ions are detected by a mass spectrometer; the combination of elution time and charge-to-mass ratio can be used to uniquely identify the mixture component. Ion mobility spectrometry, in which a sample is ionized and then allowed to drift through a stationary gas in an electric field, is significantly more sensitive, and is the industry standard when there are a small number of very dilute targets of interest (e.g., for bomb sniffing applications). However, ion mobility spectroscopy is not generally well suited to resolving complex mixtures. In general, gas mixture analysis techniques operate at or near the 10−9 level, although sensitivities as low as 10−12 have been achieved for a few specific molecules.
UV spectroscopy of room temperature or warmer gas-phase samples is generally ineffective at resolving complex mixtures. Both absorption and emission spectra are composed of hundreds or thousands of unresolved rotational lines for each vibrational line. In many cases, this broad manifold of lines overlaps with nearby vibrational manifolds; in the case of a mixture, these broad features generally overlap with the spectrum of another mixture component. The ‘confusion limit,’ at which the spectrum of one component of a mixture overlaps with another component, is reached almost immediately.
At lower temperatures, the confusion limit is radically suppressed; rotational manifolds and hot vibrational bands are suppressed and individual rotational lines can often be resolved. Referring to FIG. 1B, narrow features in the 2 K spectrum of a naphthalene gas produced by a seeded supersonic jet are resolvable into many discrete vibrational lines, such as a peak corresponding to the strong 801 transition 100 and peak corresponding the weaker 800 transition 102. Referring to FIG. 1A, in the 300K spectrum of naphthalene gas, only a few vibronic lines can be resolved, and these features are considerably broader than the corresponding low temperature peaks. For instance, only the 801 transition 100 is resolvable, and the peak is significantly broader than the corresponding cold sample peak.
Ultraviolet/visible emissions spectra are similarly simplified at low temperature. Referring to FIG. 1C, at low temperatures (2 K) at least 10 separate vibronic lines of Perylene can be resolved in the emission spectrum (solid line 104). In the warm (290° C.) spectrum (dashed line 106), no vibronic lines can be resolved.
Microwave (rotational) absorption and emission spectroscopy of room temperature gases is generally somewhat ineffective at resolving mixtures containing large molecules (e.g., more than 10 atoms). For smaller molecules, individual rotational lines can typically be resolved, but large molecules at 300 K or above occupy so many rotational states that the rotational lines overlap and become unresolvable. This broadening is compounded when complex mixtures of many cold molecules are studied. Microwave spectra of large molecules are also drastically simplified at low temperature, as many fewer rotational states are occupied.
Samples of cold molecules with atom number higher than five have been produced using seeded supersonic jets, which produce translationally and rotationally cold molecules moving at high velocity (e.g., 300 m/s or higher). The beam produced by a seeded supersonic jet evolves spatially with a rapidly decreasing density as the molecules move farther away from the beam orifice. Seeded supersonic jets have been used for the analysis of unknown gas mixtures, but experimental sensitivity is limited by the low density and high velocity of the target gas. Typical densities in the cold portion of supersonic beams are more than 105 times lower than in the room temperature input stage of the beam. Simply cooling a warm gas-phase mixture is not generally effective; few molecules have significant vapor pressure below 200 K, and the mixture will simply condense into a liquid or a solid. Spectroscopic features of such condensed phases are generally quite broad.